Diode-pumped rare-earth-doped lasers have been known for more than twenty years. The early devices were simple extrapolation of well-known flashlamp pumped geometries. Typically a rod of laser active material, such as Nd:YAG, was side-pumped by a diode laser which merely replaced the flashlamp. Later, the laser active rods were end-pumped by laser diodes and increased efficiencies were demonstrated. The primary attractiveness of diode-pumping is that it is significantly more efficient than other forms of excitation. The other advantages include compactness and reliability.
In an effort to increase the power output new laser designs have been found that maximise the absorption of pumping radiation. To this end slab laser designs were realised which incorporated multiple passes of the pump radiation within the active medium. These devices have become known as zig-zag lasers. One form of prior art device known to the inventors is that reported by T. J. Kane, W. J. Kozlovsky and R. L. Byer in "62 dB gain multiple slab geometry Nd:YAG amplifier", Optics Letters, Vol 11, pp 216-218, 1966. In this device a complicated arrangement of external optics was required to obtain four complete round-trips in the slab. R. L. Byer and other researchers have also been responsible for developing a number of novel diode-pumped slab lasers which operate on a single transverse and longitudinal mode.
More recently, D. B. Coyle reported on a theoretical design of a zig-zag laser in "Design of a High-Gain Laser Diode-Array Pumped Nd:YAG Alternating Precessive Slab Amplifier (APS Amplifier)" in IEEE Journal of Quantum Electronics, Vol 27, No 10, 1991. The Coyle device is basically rectangular in shape with four laser diode arrays pumping the four sides of the rectangle. Two of the corners of the rectangular block are cut to form entrance and exit faces of the laser beam. Despite a promising theoretical analysis this device failed to achieve the expected efficiencies due to inefficient coupling of the pump power into the slab.
A further form of prior art known to the inventors are devices sold by Spectra-Physics laser Diode Systems known as Tightly Folded Resonator (TFR) lasers. In the Spectra-Physics device a slab of Nd:YLF is close coupled to a 10-W laser diode bar by a fibre lens. The surface of the slab closest to the diode is coated to be highly reflective at the laser wavelength of 1047 nm and highly transmissive at the pump wavelength of around 800 nm. The opposite side of the slab has an AR/HR step coating. The highly reflective surface forms the tightly folded resonator whereas the anti-reflective coating forms the output coupler. The effective cavity length of the Spectra-Physics device is claimed as 30 cm. The TRF is best suited for CW pumping as amplified spontaneous emission (ASE) and associated parasitic oscillation is a limitation with the higher inputs of Quasi-CW pumping.
While the Spectra-Physics device shows high slope efficiency the maximum power obtainable from the design is limited by super-radiance occurring between the parallel opposite reflective coatings. Furthermore, the multiple-pass optical path is quite complicated due to the requirement to match the reflection points to the active areas of the laser diode.
It is an object of the present device to provide a diode pumped slab laser which overcomes one or more of the above identified deficiencies in the prior art.
It is a further object of the invention to provide a laser having a thin, compact slab with a sufficient length to absorb a large fraction of the pump energy without suffering from super-radiance problems.
It is a still further object of the invention to provide a laser having a multi-folded path within a slab that allows a small beam to sweep out the full volume of the slab at least twice.